Method and device for identification of a type of material in an object and utilization therefor

ABSTRACT

A method and apparatus for identifying a material type in an article, such as wholly or partly transparent bottle of plastic or glass, including, with the aid of a detector station, irradiating the article with rays from an infrared radiation source, detecting rays which have passed through the article non-absorbed, and then carrying out a correlation analysis of such detected rays. The article is made to pass through the detector station in a continuous or discontinuous movement. The rays from the radiation source are successively filtered by the filters consisting of wholly or partly transparent materials which have different spectral characteristics. The rays filtered by the filters and non-absorbed by the article are intercepted in order to form a sequence of measured values representing characteristic transmission signatures of the article. A correlation analysis of the signatures is carried out in relation to statistical models in order to determine the material type of the article.

The present invention relates to a method and apparatus for identifyinga material type in an article, such as a wholly or partly transparentbottle of plastic or glass, comprising, with the aid of a detectorstation, irradiating the article with rays from an infrared radiationsource, detecting rays which have passed through the articlenon-absorbed, and then carrying out a correlation analysis of suchdetected rays, as is set forth in the preamble of the enclosed patentclaims.

The invention also relates to a use of the method and apparatus in areverse vending machine for identifying and sorting bottles of differenttypes of material.

Identification of different material types, and especially types ofplastics, is a problem area of growing importance, partly as a result ofmaterials recycling becoming a progressively higher priority, from botha private financial and a socioeconomic point of view. If collectionprogrammes for used materials are to be of maximum profitability, it isessential to ensure that the materials are as pure as possible as earlyas possible in the collection and handling chain. Pure materials have avalue as raw materials for re-use, and there is a well-developed marketand industry which handle such materials. If the materials are not pure,payment must often be made for their disposal.

A number of methods for identifying different types of plastics arepreviously known. A reliable method, which is frequently used, isspectroscopy in the infrared range. All known instruments for suchspectroscopic identification of plastics are costly, as both thewave-length selecting elements (e.g., filters or gratings) and also theinfrared sources and detectors are expensive.

As further illustration of the prior art in connection with, inter alia,the use of spectroscopy and infrared light, reference is made to U.S.Pat. Nos. 5,512,752, 4,719,351, 5,206,510, German patent publications DE19601923, 19543134 and 4340795, and the Japanese patent applicationsJP-A-9138194, 6288913 and 6210632.

A number of other methods and equipment for the detection of plasticshave also been developed which are somewhat cheaper than spectroscopicmethods and equipment, but where the result of the detection is lessreliable. Examples of such known equipment are the triboelectricdetector and the optical double refraction detector.

Furthermore, the use of so-called correlation spectroscopy is known inconnection with the measurement of gases, both for the detection ofgases and concentration measurement. The gas to be analysed is used as afilter.

The object of the present invention is to use a similar technique forthe detection of plastic materials. Absorption spectra of solidsubstances such as plastics are very different from the absorptionspectra of gases. Whilst gases have very many, very fine lines in thespectrum, plastic materials have fewer and broader lines, so that as arule the spectra of different materials have more or less overlappinglines. In such a situation, more information is obtained about thematerial to be identified by measuring the degree of spectraloverlapping with a number of different plastic materials.

The method mentioned above is characterised, according to the invention,by

causing the article to pass through the detector station in a continuousor discontinuous movement;

causing the rays from the radiation source to be filtered successivelyby filters consisting of wholly or partly transparent materials havingdifferent spectral characteristics;

intercepting the rays filtered by the filter and non-absorbed by thearticle to form a sequence of measured values representingcharacteristic transmission signatures of the article; and

carrying out a correlation analysis of the signatures in relation tostatistical models in order to determine the material type of thearticle.

Similarly, the apparatus mentioned above is characterised by

a conveyor controlled by a means to cause the article to movecontinuously or discontinuously and to pass through the detectorstation;

a device in the detector station provided with a plurality of filtersconsisting of wholly or partly transparent materials having mutuallydifferent spectral characteristics to effect successive and differentfiltration of the rays from the radiation source;

a means arranged to intercept the rays filtered by the filter andnon-absorbed by the article and form a sequence of measured valuesrepresenting characteristic transmission signatures; and

an analyser, e.g., a microprocessor, which is adapted to carry out acorrelation analysis of the signatures in relation to statistical modelsin order to determine the material type of the article.

Here, statistical models should be understood to mean pre-establishedreference values, so-called calibration values which are related tosignatures of certain types of material.

Additional embodiments of the method and the apparatus are set forth inthe attached patent claims and in the description below with referenceto the attached drawings.

As mentioned above, an advantageous application of the method andapparatus would be to use them in a reverse vending machine to identifyand sort bottles of different material types.

The invention will now be described in more detail with reference to theattached drawings which show embodiments that do not define the limitsof the invention.

FIG. 1 illustrates the principle of the apparatus according to thepresent invention.

FIG. 2 is a perspective view of part of the apparatus shown in FIG. 1.

FIG. 3 shows a filter device for use with the apparatus according to theinvention.

FIG. 4 is a top view of an alternative embodiment of a filter device andits location in a detector station.

FIG. 5 shows a measured light signal after preliminary treatment inorder to give transmission signatures as a function of time for a numberof different materials.

FIG. 6 shows in the form of a flow diagram the series of evaluationswhich a processor, according to the invention, must make.

FIG. 7 shows test result of the spectral transmission of selected filtermaterials, multiplied by the transmission of an envelope filter.

FIG. 8 shows average transmission curves for types of plastics whichhave been identified in a test installation.

FIG. 9 shows a modification of the filter device in FIG. 3.

FIG. 10 shows a modification of FIG. 5 by using the filter device ofFIG. 9.

The apparatus according to the invention has a detector station,including an infrared radiation source 1 which has a hot element 2 thatemits infrared rays 3, optionally via a mirror 4. The radiation source 1has an illuminator aperture 5 and the infrared beam is guided towards afilter device 6, preferably consisting of a rotatable, round orpolygonal disc 7 which via a rotation shaft 8 is caused to rotate by amotor 9 which via cable connection 12, 13 is supplied with electricdrive current under the control of a microprocessor 11. The light filterdevice 6 is provided with a plurality of different light filters 10. Ina preferred test version, as shown in FIG. 3, the chosen number of lightfilters is ten.

In order to provide a wavelength limitation, it would be advantageous toprovide an envelope filter 14 between the illuminator aperture 5 of theradiation source 1 and the filter device 6. Furthermore, it is alsoadvantageous to position a diaphragm 15 in the path of the light beam.The filters 10 provided on the filter device in the form of a wheel or arotating disc consist at least in part of a number of light-transparentpieces of different types of plastic material, e.g., selected from thematerial group consisting of polyethylene terephthalate, polyethylenenaphthalate, polyvinyl chloride, polypropylene, polyethylene,polystyrene, acrylonitrile-butadiene-styrene copolymer, polymethylmethacrylate, polyamide, polyurethane, polysulphonate and polycarbonate.In addition, at least one of the filters may optionally be of glass,e.g., Pyrex® glass.

Although FIG. 1 shows the order of the envelope filter 14, the filterdevice 6 and the diaphragm 15 preferred at present, it should beappreciated that their position relative to one another may dedifferent. Similarly, it is conceivable that one or more of thesecomponents may, e.g., be positioned on the opposite side of the conveyor17. Furthermore, it is possible that, e.g., the diaphragm 15 and theenvelope filter 14 may made in the form of a single unit, or that thefilter device 6 and the diaphragm 15 may be combined into one unit.

In FIG. 3 the filters 10 used in the filter device are indicated by thereferences 10 _(n) and where in the chosen example n=1 . . . 10.However, it will be appreciated that more or fewer filters are possiblewithin the scope of the invention, as will also be referred to inconnection with FIG. 4.

Of the filters shown in FIG. 3, the filter 10 ₁ forms a reference filterwhich preferably is made of a spectrally uniform or material-freediaphragm 16 (see FIG. 2). The filter 10 ₂ is of an opaque material,e.g., completely black, thus preventing the passage of light raystherethrough. Due to its area-limiting light ray penetrability throughthe diaphragm or aperture 16, the filter 10 ₁ produces a signal peakreference value in the sequences of measured values. The light rayimpenetrability of the filter 10 ₂ will create a trough reference valuein the sequence of measured values.

In the test installation, the other filters 10 ₃, 10 ₄, 10 ₅, 10 ₆, 10₇, 10 ₈, 10 ₉ and 10 ₁₀ were chosen from materials consistingrespectively of polycarbonate (PC), polystyrene (PS), polyethyleneterephthalate (PET), glass (Pyrex® glass), polyvinyl chloride (PVC),polyethylene (PE), polypropylene (PP) and polyethylene with an appliedfilm, here designated UK21. After the light rays have passed thediaphragm 15, they will, possibly somewhat more concentrated(collimated) than indicated in FIG. 1, pass over a conveyor 17 and thento a detector 18 with a lens 18′, optionally via a focusing mirror 19.

In those cases where the article to be detected is a bottle, the bottlemay either be transported in an upright position as indicated by meansof the reference numeral 20, or in a horizontal position as indicated bymeans of the reference numeral 21. The conveyor 17 is driven via drivingroller 22 from a motor 23, and the operation of the motor can becontrolled from the microprocessor 11 via a control cable 24. If it isdesirable to cause the article to be detected to stop in the detectionzone or optionally to pass through at a reduced speed, this can becontrolled from the microprocessor 11 via the output 24 to the motor 23.The conveyor 17 may be of any type. If the bottle is transported in ahorizontal position, as indicated by means of the reference numeral 21,the conveyor may, for instance, consist of spaced continuous cords orwires.

The conveyor 17 may move either continuously or discontinuously. Theconveyor may also conceivably be a rotating plate which is drivencontinuously or discontinuously.

It is also conceivable that the bottles could arrive at the detectionzone (i.e., in the light beam 3) at such intervals that the bottle for abrief instant can be held motionless there. If the conveyor 17, forinstance, is not a belt-based conveyor, but a tube or chute, the bottlecould conceivably be held motionless for a brief instant in thedetection zone, so that determination of the material type of the bottlemay easily be made in that the light rays pass into the transportchannel or chute through an opening therein and pass out through anopening on the other side of the chute.

By causing the filter device 6 with its disc 7 containing the filters 10to rotate, the filters will in turn come into the light transmissionpath of the light rays 3. A sequence of signal pulses of varyingintensity, one for each of the filters used, will thus be emitted fromthe detector 18. The signal intensity of the measured values in thesequence will be dependent upon the material type in the article underexamination, and will moreover be highly characteristic for each type ofmaterial, especially when using plastic materials typical for thearticle.

Articles examined in this way will, together with the successivefilters, bring about a sequence of characteristic transmissionsignatures for each individual article.

The present invention could advantageously be used for identifying anumber of typical plastic materials, such as polyethylene terephthalate(PET), polyethylene naphthalate (PEN), polyvinyl chloride (PVC),polyethylene (PE), polypropylene (PP), polycarbonate (PC), polystyrene(PS), acrylonitrile-butadiene-styrene copolymer (ABS), polymethylmethacrylate (PMMA), polyamide (PA), polyurethane (PUR), polysulphonate(PSU) etc. The same materials as those which are to be identified areused as filter materials for the filters 10 (see the filters 10 ₃-10 ₁₀in FIG. 3). The selectivity attainable will be further increased, themore filters and filter materials that are used. A filter device 6 suchas that shown in FIGS. 1, 2 and 3, will potentially be very inexpensiveto manufacture, as the plastic materials used as filters are veryinexpensive, and at the same time they allow the passage of radiation ina wide wavelength range. This makes total transmitted radiation high,which in turn allows use of detectors 18 of such types that areinexpensive, for example, pyroelectric detectors, thermoelectricdetectors or uncooled photoconductors such as PbS and PbSe. It will bepossible to optimise selectivity of the method and apparatus accordingto the invention by limiting the spectral range used to a wavelengthrange wherein the materials used have different characteristic spectralfeatures or transmission signatures. A limitation of this kind may bemade using band pass filters, and an example of such a filter isindicated by the reference numeral 14 in FIG. 1. The wavelength rangemay, e.g., extend from about 3 micrometers to about 4 micrometers,preferably from 2 to 2.5 micrometers or alternatively from about 1.6 to1.8 micrometers.

Of course, other wavelength ranges are also conceivable in connectionwith possible other plastic materials which are to be detected.

The repeated sequence of signal pulses emitted from the detector 18 canbe seen from FIG. 5 as a typical example. These signal pulses are passedto an analog/digital converter 25, from where signals are passed to themicroprocessor 11. As shown in FIG. 5, the reference filter 10, willprovide a distinct reference pulse designated “Ref” on FIG. 5. Theopaque filter material 102 will produce the signal valley which isindicated by the reference Opaque. The signal spectrum thus contains areference signal peak caused by the area-limited, unfiltered light, andthe reference signal trough that is due to a blocking of the light rays.The microprocessor 11 will analyse successively each of the other signalpeaks in the sequence of measured values, e.g., the signal peak 27 (FIG.5) relative to a mean value 28 of two adjacent signal valley.

On the basis of the measurements taken, it will be possible to compute alight transmission value for an article based on the level value 26 of arespective signal peak minus the said mean value. The said values mayoptionally be determined on the basis of a normalised signal intensity,as can also be seen from FIG. 5. As FIG. 5 shows, in a testinstallation, it was possible to carry out a scanning cycle in thecourse of about 70 milliseconds. The microprocessor II operates so thaton the computation of the light transmission values it collects thesequence of the computed light transmission values from a signal cyclein a vector consisting of n elements, wherein n is equal to the numberof filters, and compares this with corresponding measured values for asubsequent signal sequence or scanning sequence.

The microprocessor 11 is capable of computing the average value of twosuccessive signal sequence values and deriving with the aid of acalibration or identification operation, e.g., PLS (Partial LeastSquares) discriminant analysis, a unique characteristic of the materialtype of which the article is made. This calibration and identificationoperation includes use of a calibration and identification algorithm.

Although the design of the filter device shown and described inconnection with FIGS. 1-3 may be typically like that shown in FIGS. 2and 3, it is quite conceivable that the filters 10 arranged on thefilter device 6 may be positioned closer together.

As an alternative, it is possible that the filter device may have theappearance of a drum-like structure, e.g., produced using extrudedaluminium or plastic. The drum 31, in the proposed exemplary embodiment,may have a vertical or horizontal rotation shaft, depending upon thedesign and position of the radiation source 1. The drum 31 is supportedby arms 32 which are secured to the rotation shaft 34 of the drivingmotor 33. The said filters can be placed in open sections 31′ in thewall of the drum. By dividing the drum wall up as shown in FIG. 4, itwill be possible to position a total of 16 filters. However, it will beappreciated immediately that it will be possible to position a larger orsmaller number of filters on the drum, depending upon the number offilter openings that are provided. The drum may be circular incross-section, or optionally have a polygonal cross-section.

It is also conceivable that the drum 31 may be positioned on thedetector side of the conveyor 17, e.g., rotating about the detector 18instead of about the radiation source 1.

The motor 33 which turns the drum 31 may, e.g., be a DC motor like themotor 9 in FIG. 1

When an article, such as a bottle 20 or 21, is placed in the light pathbetween the radiation source 1 and the detector 18, for each revolutionof the filter device, an irradiation is carried out with n differentspectral characteristics, depending upon the number of filters used. Thetransmitted radiation is measured continuously by the detector. A signalamplifier can be installed in the detector and through the analysis ofthe transmission values it will be possible to make a very reliableclassification of the different types of plastic. In a rudimentary testof the present invention, when testing a limited number of articles, anaccuracy of 80% was attained, whilst 16% could not be classified and 4%were incorrectly classified. However, it should be understood that arefining of the calibration or identification algorithm and theequipment used will increase the accuracy of the measurements.

The filters used in the filter device 6 consist typically of bits ofsectors of a circle. In addition to the various polymers, Pyrex® glasswas found to be suitable as a filter material. The spectral transmissionof the filter materials multiplied by the transmission of the envelopefilter 14 can be seen in more detail in FIG. 7. However, the filtermaterials shown in this figure must not be perceived as in any waydefining the limits of the application of the invention.

FIG. 8 shows the average transmission curves for each of the types ofplastic in question which were identified during the preliminary tests.

According to a prototype of the invention, the envelope filter 14limited the wavelength range to 2.9-3.8 micrometers. However, it will beappreciated that other wavelength ranges may conceivably be employed byusing other filters. The diaphragm 15, which determines how muchradiation is to pass through and be sent towards the article, will alsoensure that light rays pass through from only one filter at a time. Thediaphragm may have a fixed or variable diameter, and an appropriatediameter has been found to be 13 mm, although this must not be perceivedas in any way defining the limits of the invention.

In FIG. 2, for the purposes of illustration, it is supposed that theradiation which strikes the article, such as a bottle 20 or 21, is notdeflected therein. However, in practice, light rays will be refracteddepending on the angle and where on the bottle they fall. However, someof the light rays which pass through the article will always strike thedetector 18, optionally via the mirror 19.

In particular when detecting bottles, it would be advantageous totransport these in a horizontal position and concentrate the detectionarea on the bottle on its neck portion.

A simple amplifier with high and low pass filters may be built into orconnected to the detector 18. In a test installation, the signal fromthe detector was given a sequence of about 170 Hz and thus periodicallyhad a frequency of 170/10 equalling 17 Hz (because of the 10 filters inthe filter device 6). For each measurement, 4-5 periods were sampled,whereupon a digital smoothing and normalising was carried out, thesignal frequency was computed and the reference point (signal maximum)was localised. FIG. 5 shows the signal as it appeared subsequent to thisprocess.

As previously indicated, the transmission values are computed as thelevel of the respective signal peak, minus the mean value of two“neighbouring troughs”. The computed values are collected in a vector of10 elements and are compared with corresponding values for a subsequentsignal sequence. If the signal varies excessively, the measurement isrejected, but if the signal is acceptable, average values are computedand then passed on to the calibration and identification algorithms.

In FIG. 1 reference numeral 29 indicates peripheral equipment associatedwith the microprocessor 11, such as equipment for paying a returndeposit for articles received, such as returned bottles, and for givinginformation to a person operating the apparatus if it is a part of areverse vending machine. The reference numeral 30 indicates yet moreperipheral equipment, such as after-treatment equipment in the form of,for instance, sorting devices, compactors, material cutters, additionalconveyors etc.

In FIG. 6, by way of a summary, the process steps which must be carriedout to classify an article entering a detector station are indicated ingeneral terms.

Block 35 indicates that the system is waiting for an article to enterthe radiation path. When such an article is present, block 36 indicatesthat measurement data are fed into the detector 18 with subsequentdigital signal processing in the A/D converter 25 and with computationof transmission values with the aid of the microprocessor 11. Thedecision block 37 indicates that the microprocessor 11 considers whetherthe sequence of the received and analysed measured values vary overtime. If such is the case, measured values must be fed in and processedagain. However, if such is not the case, a new decision will be taken asrepresented by block 38 with respect to whether the signal level iswithin an acceptable, predetermined range. If this is not the case, asindicated by block 39, e.g., that the article of which a measurementtest is taken is too thick or too thin, the measurement will be rejectedand the article deemed non-accepted. In such a case, it is possible, viathe peripheral equipment as indicated in FIG. 1 by means of referencenumeral 34, to convey the article received back to the person who hasinserted it, or optionally to convey the article to a receptacle forunidentified articles.

However, if the signal level is in an acceptable range, the article willbe classifiable, as indicated at block 40, thus allowing the article tobe conveyed to the correct further treatment, whether this be compactionof the article or cutting it up, or to re-use of the article. This isgenerally indicated by reference numeral 41, which also includes thepossibility, via the peripheral equipment 29, of printing out a receiptfor the person who inserted the article showing the return depositvalue, if any, of the article.

During the testing of a prototype of the apparatus according to theinvention, the calibration was done by means of a method which isgenerally referred to as PLS discriminant analysis, wherein PLS in thisconnection stands for “Partial Least Squares”, and is a method used inthe calibration of instruments with many wavelength ranges, and whereinthe individual wavelength ranges may be correlated. This type ofanalysis is well suited for distinguishing between two fractions. One ofthe fractions can be given a y value of +1, whilst the other fractioncan be given a y value of −1, and the PLS analysis can then be used tofind an optimal regression vector which distinguishes the two fractionsunder given conditions. In the case where, e.g., ten filters are used,as shown in FIG. 3, all the computation necessary to find out whichfraction an unknown sample belongs to will be to multiply the regressionvector by the sequence of measured values which are obtained as the tenfilters pass by. In this specific instance, eight regression vectors arecomputed, wherein the first divides the multidimensional space in two.By examining the decomposition with the aid of an analysis procedurethat is known per se, it is seen that PC, PEN and PET constitute onefraction, whilst PE, PP, PS and PVC constitute a second fraction of thetypical wavelength range of 2.8-3.9 micrometers. After PC, PEN and PEThave been separated as a fraction, a regression vector is computed whichseparates PEC, one which separates PEN and one which separates PET fromthis fraction. Similar methodology is used for the other types ofplastics.

This method is simple per se and will give satisfactory results, but adrawback of PLS discriminant analysis is that it attempts to plot allarticles of one type as the integer 1, whilst all other types areplotted as the integer −1. The regression vector used is therefore notquite optimal, even though the method of analysis is simple. Use of PLSdiscriminant analysis is therefore only named as a possible method ofanalysis. Other possible methods of analysis are, e.g., “PrincipalComponent Analysis”, direct correlation, Mahalonobis'discrimination,neural network analysis and “fuzzy” logic.

As mentioned above, an article which is to be detected is transportedthrough the detector station in a continuous or discontinuous movement.The discontinuous movement may, e.g., mean that the article, when itcomes into the path of the light beam 3, is made to stop briefly once oroptionally several times with intervening small movements. If anarticle, e.g., a bottle, is furnished with large labels, it may beexpedient to cause the article to rotate in the detector station until amaximum signal intensity through the reference filter, such as thefilter 10′ in FIG. 3, is registered. In such a case, it would be ofadvantage if the article, such as a bottle, were transported in ahorizontal position, as indicated by the reference numeral 21, and thatin the detector station there is equipment for rotating the article, inthe form of peripheral equipment belonging to the block 33.

In a preferred execution of the invention, the neck of the bottle inparticular will be suitable for material detection. FIG. 9 shows amodification of the filter disc 7 which can be seen in FIG. 3. On thisFIG. 8 shows a modification of the filter disc 7 which can be seen inFIG. 3. On this chosen version of the filter-carrying disc that does notdefine the limits of the invention, there is provided a total of eightfilters 10, of which the filter 10 ₁ is a completely transparentreference filter which preferably, but not necessarily, is made of adiaphragmed but material-free (i.e., open) aperture 16 (see FIG. 2). Theother filters 10 ₂, 10 ₃, 10 ₄, 10 ₅, 10 ₆, 10 ₇ and 10 ₈ could beselected from among, e.g., the following materials: polycarbonate (PC),polystyrene (PS), polyethylene terephthalate (PET), glass (Pyrex®glass), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP)and polyethylene. Between the adjacent filters 10 ₁, 10 ₂, 10 ₃, 10 ₄,10 ₅, 10 ₆, 10 ₇ and 10 ₈ there are arranged respective filter-freediaphragms, or diaphragms of the same filter material in all, ordiaphragms of limited light area, indicated by 42 ₁, 42 ₂, 42 ₃, 42 ₄,42 ₅, 42 ₆, 42 ₇ and 42 ₈, respectively. The diaphragms or apertures 42will help to produce measured reference values between the measuredvalues of the spectral signatures in the sequence of measured valuesobtained when light rays successively pass through the filters 10. Thiswill make the determination of such successive measured values moreexact because there are always reference values on each side of thesignature measured value.

If it is desirable to measure the rotation of the disc 7′, the signalobtained via the filter 10 ₁ will produce a start reference (greateramplitude than that obtained via the apertures 42 ₁, 42 ₂, 42 ₃, 42 ₄,42 ₅, 42 ₆, 42 ₇ and 42 ₈), whilst the diaphragms or apertures 42 willproduce subsequent position indication for the subsequent signaturemeasured values and also indicate the rate of rotation of the disc 7′(number of pulses from the diaphragms/apertures 42 per unit of time).

What is claimed is:
 1. A method for identifying a material type in anarticle, including a wholly or partly transparent bottle of plastic orglass, comprising, with the aid of a detector station, irradiating thearticle with rays from an infrared radiation source, detecting rayswhich non-absorbed have passed through the article, and then carryingout a correlation analysis of such detected rays, characterised bycausing the article to pass through the detector station in a continuousor discontinuous movement; causing the rays from the radiation source tobe successively filtered by filters consisting of wholly or partlytransparent materials having different spectral characteristics;intercepting the rays filtered by the filter and non-absorbed by thearticle in order to form a sequence of measured values representingcharacteristic transmission signatures of the article; and carrying outa correlation analysis of the signatures in relation to statisticalmodels in order to determine the material type of the article.
 2. Amethod as disclosed in claim 1, characterised by causing the filters tomove along a circular path.
 3. A method as disclosed in claim 1,characterised by at least one of the filters is of a material selectedfrom the group consisting of polyethylene terephthalate, polyethylenenaphthalate, polyvinyl chloride, polypropylene, polyethylene,polystyrene, polycarbonate, acrylonitrile-butadiene-styrene copolymer,polymethyl methacrylate, polyamide, polyurethane, polysulphonate, Pyrex®glass, or made as a diaphragmed, material-free aperture.
 4. A method adisclosed in claim 1, characterised in that two of the filters areselected to give respectively area-limited light ray penetrabilitythrough a diaphragm or aperture to produce a signal peak reference valuein the sequence of measured values, and light ray impenetrability toproduce a valley reference value in the sequence of measured values. 5.A method as disclosed in claim 1, characterised in that in the sequenceof measured values there are produced reference measured values whichare adjacent to the respective signature measured values.
 6. A method asdisclosed in claim 1, characterised by providing a wavelength limitationin the radiation path between the radiation source and the detector. 7.A method as disclosed in claim 1, characterised by providing a partialdiaphragming of the rays in the radiation path between the radiationsource and the detector.
 8. A method as disclosed in claim 1,characterised by deflecting the light rays passing through the articletowards the detector by means of a focusing mirror.
 9. A method asdisclosed in claim 1, characterised by providing in the sequence ofmeasured values at least one reference signal peak caused byarea-limited, material-unfiltered light, and analyzing successively eachof the other signal peaks in the sequence of measured values in relationto a mean value of two adjacent signal valleys.
 10. A method asdisclosed in claim 9, characterised by providing in the sequence ofmeasured values at least one reference signal valley caused by blockingthe light rays; and analyzing each of the other signal valleys in thesequence of measured values in order to form successively said meanvalue of two adjacent signal values in relation to successive signalpeaks.
 11. A method as disclosed in claim 9, characterised by computinga light transmission value for an article based on a level value of arespective signal peak minus said mean value.
 12. A method as disclosedin claim 11, characterised by collecting the computed light transmissionvalues form a signal cycle in a vector consisting of n elements, whereinn is equal to the number of filters, and comparing them withcorresponding values for a subsequent signal period.
 13. A method asdisclosed in claim 12, characterised by rejecting an article measurementif the difference between two successive signal period values exceeds athreshold within an otherwise acceptable measurement sequence.
 14. Amethod as disclosed in claim 13, characterised by computing the averagevalue of two successive signal period values, and deriving with the aidof a calibration and identification operation, e.g., PLS (Partial LeastSquares) discriminant analysis, a unique characterisitic of the materialtype of which the article is made.
 15. A method as disclosed in claim14, characterised in that the calibration and identification operationincludes the use of a calibration and identification algorithm.
 16. Useof a method as disclosed in claim 1, in a reverse vending machine foridentification and sorting of bottles of different material types. 17.An apparatus for identifying a type of material in an article, includinga wholly or partly transparent bottle of plastic or glass, comprising,with the aid of a detector station, irradiating the article with raysfrom an infrared radiation source, detecting rays which have passedthrough the article non-absorbed, and then carrying out a correlationanalysis of such detected rays, characterised by a conveyor controlledby a means to cause the article to move continuously or discontinuouslyand to pass through the detector station; device in the detector stationprovided with a plurality of filters consisting of wholly or partlytransparent materials having different spectral characteristics toeffect successive and different filtration of the rays from theradiation source; a means arranged to intercept the rays filtered by thefilter and non-absorbed by the article in order to form a sequence ofmeasured values representing characteristic transmission signatures ofthe article; and an analyzer including a microprocessor, adapted tocarry out a correlation analysis of the signatures in relation tostatistical models in order to determine the material type of thearticle.
 18. An apparatus as disclosed in claim 17, characterised inthat the filters are arranged on a rotatable, round or polygonal disc inorder to move along a circular path.
 19. An apparatus as disclosed inclaim 17, characterised in that the filters are arranged in the wall ofa drum which is rotatable about an IR radiation source or the detector,and that the drum has a circular or polygonal cross-section.
 20. Anapparatus as disclosed in claim 17, characterised in that at least oneof the filters is of a material selected from the group consisting ofpolyethylene terphthalate, polyethylene naphthalate, polyvinyl chloride,polypropylene, polyethylene, polystyrene,acrylonitrile-butadiene-styrene copolymer, polymethyl metharcrylate,polyamide, polyurethane, poly sulphonate, polycarbonate and Pyrex®glass.
 21. An apparatus as disclosed in claim 17, characterised in thattwo of the filters are of an opaque material and in the form of adiaphragmed, but material-free aperture, respectively.
 22. An apparatusas disclosed in claim 17, characterised in that an aperture is providedbetween adjacent filters.
 23. An apparatus as disclosed in claim 22,characterised in that each of the diaphragms is made in the form of adiaphragmed aperture or aperture having light-transparent material thatis the same for all the apertures.
 24. An apparatus as disclosed inclaim 17, characterised in that a wavelength limiting envelope filter isprovided in the radiation path between the radiation source and thedetector.
 25. An apparatus as disclosed in claim 24 characterised inthat the processor is adapted to compute the average value of twosuccessive signal sequence measured values, and carry out an analysiswith the aid of a calibration and identification operation, e.g., PLS(Partial Least Squares) discriminant analysis, in order to give a uniquecharacteristic of the material type of which the article is made.
 26. Anapparatus as disclosed in claim 17, characterised in that a diaphragm isarranged in the path of a light beam.
 27. An apparatus as disclosed inclaim 17, characterised in that in association with the detector thereis provided a focusing mirror for guiding the light rays which passthrough the article to the detector.
 28. An apparatus as disclosed inclaim 17, characterised in that the processor is adapted to register atleast one reference signal peak in the sequence of signature measuredvalues from the detector caused by unfiltered light.
 29. An apparatus asdisclosed in claim 28, characterised in that the processor is alsoadapted to register at least one reference signal valley in the sequenceof measured values of signals caused by blocking the light rays, andthat the processor also has an analyzer for successively analyzing eachof the other signal peaks in the sequence of measured values in relationto a mean value.
 30. An apparatus as disclosed in claim 29,characterised in that the processor is adapted to compute the lighttransmission value for an article based on a level value of a respectivesignal peak minus said mean value of two adjacent signal valleys.
 31. Anapparatus as disclosed in claim 28, characterised in that the processoris adapted to compute the transmission signature value for an articlebased on the level of a respective signal peak in relation to the valueof an adjacent reference signal peak.
 32. A method as disclosed in claim28,characterised in that the processor is adapted to compute the averagevalue of two successive signal sequence measured values, and carry outan analysis with the aid of a calibration and identification operation,e.g., PLS (Partial Least Squares) discriminant analysis, in order togive a unique characteristic of the material type of which the articleis made.
 33. An apparatus as disclosed in claim 17, characterised inthat the processor is adapted to collect the computed transmissionsignature measured values from a sequence of measured values in a vectorconsisting of n elements, wherein n is equal to the number of filters,and compare with corresponding values for a subsequent signal period.34. An apparatus as disclosed in claim 17, characterised in that theprocessor is adapted to reject an article measurement if the differencebetween two successive signal sequence measured values exceeds athreshold.
 35. Use of an apparatus as disclosed in claim 17, in areverse vending machine for identification and sorting of bottles ofdifferent material types.